1. Field of the Invention
The present invention relates to a resin-filled carrier used in a two-component electrophotographic developer used in copiers, printers and the like. More specifically, the present invention relates to a resin-filled carrier for an electrophotographic developer having a high breakdown voltage and a high particle breaking strength, and an electrophotographic developer using this resin-filled carrier.
2. Description of the Related Art
Electrophotographic developing methods develop by adhering toner particles in a developer to an electrostatic latent image which is formed on a photoreceptor. The developer used in such methods can be classified as either being a two-component developer composed of toner particles and carrier particles, or a one-component developer which only uses toner particles.
Among such developers, as a developing method using a two-component developer composed of toner particles and carrier particles, a cascade method or the like has long been employed. However, currently magnetic brush methods using a magnet roll have become mainstream.
In a two-component developer, carrier particles act as a carrying substance for imparting the desired charge to the toner particles and transporting the toner particles thus-imparted with a charge to the surface of the photoreceptor to form a toner image on the photoreceptor by stirring the carrier particles with the toner particles in a developing box which is filled with the developer. Carrier particles remaining on the developing roll which supports the magnets return back into the developing box from this developing roll, and are then mixed and stirred with new toner particles for reuse over a certain time period.
Unlike one-component developers, in two-component developers the carrier particles are mixed and stirred with the toner particles to charge the toner particles. The carrier particles also have a transporting function and are easily controlled when designing the developer. Therefore, two-component developers are suitable for full color developing apparatuses in which high image quality is demanded and for apparatuses performing high-speed printing in which the reliability and durability of image sustainability are demanded.
In two-component developers which are used in such a manner, the image properties, such as image density, fogging, white spots, gradation and resolution, need to exhibit a certain value from the initial stage. Furthermore, these properties must not change during printing and have to be stably maintained. To stably maintain these properties, it is necessary for the properties of the carrier particles in the two-component developer to be stable.
Conventionally, various kinds of carrier, such as an iron powder carrier, a ferrite carrier, a resin-coated ferrite carrier, a magnetic powder-dispersed resin carrier and the like, have been used for the carrier particles forming a two-component developer.
In recent years the workplace has become more networked, evolving from an era of single-function copiers to multifunction devices. In addition, the type of service provided has shifted from a system in which a contracted repair worker carries out regular maintenance and replaces the developer and other parts to a maintenance-free system. Further, demands from the market for even longer developer life are becoming much greater.
In view of these circumstances, Japanese Patent Laid-Open No. 5-40367 proposes many magnetic powder-dispersed carriers in which fine, magnetic microparticles are dispersed in a resin to extend developer life by making the carrier particles lighter.
Such a magnetic powder-dispersed carrier can reduce true density by reducing the amount of magnetic microparticles, thus reducing the stress from stirring. As a result, chipping or peeling of the coating can be prevented, whereby stable image properties for a long period of time can be obtained.
However, because a binder resin covers the magnetic microparticles, the magnetic powder-dispersed carrier has a high carrier resistance. Thus, there is the drawback that it is difficult to obtain sufficient image density.
In addition, since the magnetic microparticles are hardened by the binder resin, the magnetic powder-dispersed carrier has also had the drawbacks that the magnetic microparticles detach due to stirring stress or from shocks in the developing apparatus, and that the carrier particles themselves split, possibly as a result of having inferior mechanical strength as compared with the conventionally-used iron powder carrier or ferrite carrier. The detached magnetic microparticles or split carrier particles adhere to the photoreceptor, thereby becoming a factor in causing image defects.
Further, a magnetic powder-dispersed carrier has the drawback that since fine magnetic microparticles are used, remnant magnetization and coercive force increase, so that the fluidity of the developer deteriorates. Especially when a magnetic brush is formed on a magnet roll, the bristles of the magnetic brush stiffen due to the presence of remnant magnetization and coercive force, which makes it difficult to obtain high image quality. There is also the problem that even when the carrier leaves the magnet roll, because the carrier magnetic agglomerations do not come unloose and the carrier cannot be rapidly mixed with the supplied toner, the rise in the charge amount is poor, which causes image defects such as toner scattering and fogging.
In addition, while a magnetic powder-dispersed carrier can be produced by two methods, crushing or polymerization, the crushing method has a poor yield, and the polymerization method has a complicated production process. Thus, both methods have the problem of high costs.
A resin-filled carrier in which the voids in a porous carrier core material are filled with a resin has been proposed as a replacement for magnetic powder-dispersed carriers. For example, Japanese Patent Laid-Open No. 11-295933 and Japanese Patent Laid-Open No. 11-295935 disclose a carrier which comprises soft-magnetic cores or hard magnetic cores, a polymer contained in the pores of the cores, and a coating which covers the cores. These resin-filled carriers enable a carrier to be obtained having few shocks, a desired fluidity, a broad range of frictional charge values, a desired conductance and a volume average particle size that is within a certain range.
Japanese Patent Laid-Open No. 11-295933 discloses that various suitable porous solid core carrier substances, such as a known porous core, may be used as the core material. Japanese Patent Laid-Open No. 11-295933 states that it is especially important that the carrier is porous and has the desired fluidity, and that soft magnetism, porosity as represented by BET surface area and volume average particle size are properties which need to be given attention.
However, as is described in the examples of Japanese Patent Laid-Open No. 11-295933, for a porosity of about 1,600 cm2/g in BET surface area, a sufficient reduction in the specific gravity is not achieved even by filling with a resin, and thus such a carrier cannot cope with the recent ever increasing demands for lengthened developer life.
Japanese Patent Laid-Open No. 11-295933 also discloses that it is difficult to precisely control the specific gravity and mechanical strength of a carrier which has been filled with resin merely by controlling the porosity as represented by BET surface area.
The measurement principle of BET surface area is to measure the physical and chemical adsorption of a specific gas, which does not correlate with the porosity of the core material. In other words, it is typical for BET surface area to change depending on particle size, particle size distribution and nature of the surface material even for a core material that has hardly any pores. Thus, even if porosity is controlled using the BET surface area measured in the above-described manner, it cannot be said that the core material can be sufficiently filled with resin. If a large amount of resin is filled into a core material having a high BET surface area value but which is not porous, or into a core material which is not sufficiently porous, the resin which could not be filled remains by itself without closely adhering to the core material. In such a state, the left-over resin floats in the carrier, causing a large amount of agglomerates to form among the particles, whereby fluidity deteriorates. When agglomerates break apart during toner usage, charge properties fluctuate greatly, making it difficult to obtain stable properties.
Further, in Japanese Patent Laid-Open No. 11-295933, a porous core is used, and the total content of the resin filled in the cores and the resin which coats the surface of the cores is preferably about 0.5 to 10% by weight of the carrier. In the examples of Japanese Patent Laid-Open No. 11-295933, the greatest total content of the resins does not even reach 6% by weight of the carrier. With such a small amount of resin, the desired low specific gravity cannot be realized, meaning that a performance that is merely approximate to that of the conventionally used resin-coated carrier is obtained.
Japanese Patent Laid-Open No. 54-78137 discloses a carrier for an electrostatic image developer in which the pores and recesses on the surface of magnetic particles, which are either porous having a bulk specific gravity that is smaller than that of a substantially non-porous substance, or which have a large surface roughness, are filled with a fine powder consisting of an electrical insulating resin.
Japanese Patent Laid-Open No. 2006-337579 proposes a resin-filled carrier formed by filling a resin in a ferrite core material having a void fraction of 10 to 60%. Japanese Patent Laid-Open No. 2007-57943 proposes a resin-filled carrier which has a three-dimensional layered structure. Japanese Patent Laid-Open Nos. 2006-337579 and 2007-57943 disclose that various methods may be employed for filling the resin in the resin-filled carrier core material, such as a dry method, a spray-dry method using a fluidized bed, a rotary-dry method, and a liquid immersion-dry method using a universal stirrer, and that a suitable method is selected according to the core material and resin to be used.
Further, Japanese Patent Laid-Open No. 2007-57943 discloses that when filling the resin, since it is difficult to fill the voids with resin under an ordinary pressure or in a pressurized state, it is preferred to reduce the pressure inside the filling apparatus, so that the resin can be efficiently and sufficiently filled in the voids inside the particles, which makes it easier to form a three-dimensional layered structure.
Further, Japanese Patent Laid-Open No. 2007-133100 discloses a carrier in which a resin is impregnated in a porous magnetic body, and a carrier coated with a large amount of resin on the surface of a core material. Since these carriers have a light true specific gravity, excess carrier can be smoothly discharged along with the toner by developing while supplying a supply developer having the toner and carrier to the developing apparatus, and as necessary using the carrier which is in excess in the developing apparatus interior in a two-component developing method supply developer which is discharged from the developing apparatus.
There are examples of the porous magnetic powder described in Japanese Patent Laid-Open Nos. 2006-337579, 2007-57943, 2007-133100 in which void volume of the core material is investigated by BET and oil absorption. However, BET only relates to surface area, and the actual level of voids cannot be found from the BET value. Further, while oil absorption does reflect void volume to a certain extent, considering oil absorption measurement principles, the gaps between the particles are also measured together with the voids in the particles, and thus oil absorption does not measure the actual void volume. Further, the gaps between the particles are usually larger than the actual void volume in the particles, so that oil absorption lacks accuracy as an index when trying to fill resin without any excess. Further, since Japanese Patent Laid-Open Nos. 2006-337579, 2007-57943, 2007-133100 do not describe the diameter of the voids which are present on the ferrite surface in which the resin is to be filled, or the distribution of such void diameters, when the resin is actually filled, there is filled resin unevenness among the particles and a lack of uniformity in resin filling. As a result, the particles which are not sufficiently filled with resin have poor strength, so that the carrier particles split and microparticles form during use in an actual machine, which are factors in image defects.
Japanese Patent Laid-Open No. 2007-218955 describes the pore size and pore volume of core material particles. Specifically, Japanese Patent Laid-Open No. 2007-218955 discloses that by providing, at the stage of the carrier core material prior to resin filling, durability capable of maintaining high resistance under high-voltage application conditions, maintenance of high-resistance during high-voltage application at the point when the carrier is used as an electrophotographic developer can be markedly improved, so that prevention of breakdown and prevention of a deterioration in image properties can be achieved. Further, Japanese Patent Laid-Open No. 2007-218955 discloses that for anti-spent properties as well, it is important to produce a porous magnetic powdered body having specific pore distribution properties, and to obtain a carrier core material by subjecting this porous magnetic powdered body to a treatment conferring high resistance.
However, it is known that in cases where both the pore distribution properties and the electrical resistance of the carrier core material are not satisfied, as in Comparative Example 4 of Japanese Patent Laid-Open No. 2007-218955, desired properties cannot be obtained.
This means that the pore distribution properties such as those described in Japanese Patent Laid-Open No. 2007-218955 are not sufficient. Thus, there is a need for a carrier core material which has more preferable pore distribution properties which are controlled more precisely.
In the above-described resin-filled carrier, there are the problems that charge tends to leak under a high voltage, breakdown voltage is low, the carrier particles may split or microparticles may be formed during strong stirring, and the particle breaking strength is low.
Thus, there is a need for a resin-filled carrier for an electrophotographic developer which, while maintaining the advantages of the above-described resin-filled carriers, has a high breakdown voltage and a high particle breaking strength.